[Paper Review] Size of the thermal source in relativistic heavy-ion collisions

Piotr Bożek|ArXiv.org|Jun 13, 2005

High-Energy Particle Collisions Research3 citations

TL;DR

This paper proposes a two-component model for relativistic heavy-ion collisions, separating the interaction region into a dense, thermalized core and a peripheral mantle. The core enhances particle production by 65% per participant pair over p+p due to thermalization and increased jet absorption, while the mantle follows wounded-nucleon scaling. The model successfully reproduces centrality-dependent particle yields and jet suppression (R_AA) across all centralities, with increased absorption in the core compatible with PHENIX data.

ABSTRACT

The dependence of the size of the thermal source on the centrality in ultrarelativistic heavy-ion collisions is studied. The interaction region consists of a well defined thermalized core, and of an outer mantle where the production scales with the number of participants. The thermal source builds up in the region with the largest density of participants in the transverse plane. Particle production in the thermalized core is enhanced in comparison to the wounded nucleon model. The change of the degree of strangeness saturation with centrality is also discussed. We perform an estimate of high pT jet absorption finding that an increase of the absorption in the thermal core is compatible with the data.

Motivation & Objective

• To explain the centrality dependence of particle production in relativistic heavy-ion collisions beyond simple wounded-nucleon scaling.
• To model the thermal source as a two-part structure: a dense, thermalized core and a peripheral, non-thermalized mantle.
• To account for enhanced particle yields and jet suppression in central collisions by attributing them to a high-density core with increased absorption.
• To reproduce experimental data on particle ratios (e.g., K⁻/π⁻) and jet suppression (R_AA) across all centralities using a spatially varying absorption model.

Proposed method

• Decomposes the interaction region into a thermalized core (where particle production is enhanced) and a mantle (where production scales with the number of participants).
• Uses a step-function density profile: ρ = (d²N_part/ds²) × [θ(d_cut - d²N_part/ds²) + (1+α)θ(d²N_part/ds² - d_cut)], with α = 0.65 for the core.
• Models jet suppression via absorption probability f = exp(–kI), where I is the path integral of density along the jet trajectory.
• Applies different absorption coefficients: k = 0.09 fm⁻¹ for the full density profile (reproducing data), and k = 0.8 fm⁻¹ for a core-only absorption model.
• Uses τ = 0.2 fm for jet formation time in standard model, τ = 0.8 fm for core-only model to reflect longer formation in dense medium.
• Compares model predictions to PHENIX data on R_AA for p⊥ > 4.5 GeV in Au+Au and Cu+Cu collisions.

Experimental results

Research questions

• RQ1 How does the size and structure of the thermal source vary with centrality in Au+Au collisions?
• RQ2 Can the observed enhancement in particle production per participant pair in central collisions be explained by a thermalized core with increased particle yield?
• RQ3 Is jet suppression in high-p⊥ probes consistent with absorption occurring primarily in a dense, thermalized core?
• RQ4 Can the centrality dependence of R_AA be reproduced by a spatially varying absorption model that includes both core and mantle contributions?
• RQ5 What is the role of the mantle in jet absorption, and does the absence of a core in peripheral collisions contradict data?

Key findings

• The thermalized core enhances particle production per participant pair by 65% compared to p+p interactions at √s = 200 GeV.
• The K⁻/π⁻ ratio as a function of centrality is well reproduced by combining contributions from a chemically equilibrated thermal core and a strangeness-enhanced mantle.
• Jet suppression (R_AA) in peripheral collisions is only reproduced when absorption occurs in both the mantle and the core, not just in the core.
• A model with increased density in the core (α = 0.65) and k = 0.09 fm⁻¹ reproduces the full centrality dependence of R_AA for p⊥ > 4.5 GeV.
• The core appears at around N_part = 40, and its presence leads to significant modifications in particle yield and fluctuations in this centrality range.
• The model successfully describes charged particle production in Cu+Cu collisions up to N_part = 120, though fluctuations in core-to-mantle size ratio may alter physics in this system.